If Japan decided going nuclear was the correct policy, her existing massive stockpile of fissile material, long and deep nuclear science infrastructure would allow her to break out quickly and build a very large stockpile of several thousand nuclear weapons, larger than the stockpile currently in service with either the US or Russia without any american assistance. Her solid fuelled satellite launch rockets have been described as virtual clones of the decommissioned MX missiles, She would also likely also be able to build her own nuclear powered submarines without out outside Help much sooner than Australia could get her american and British supplied nuclear submarines that were promised as part of Aukus months ago, if indeee Australia ever does developed the ability to support them and then get the actual submarines.
The fact is US may absolutely not wish to be seen arming Japan with nuclear weapons, but there is no doubt the US would be overjoyed if Japan were to decide to nuclearization it’s military today.
the barrier to a nuclear Japan is not the US, it is Japan. But although Japan has not chosen to turn the screw driver the final turn to actually go nuclear, her defence industry, satellite launch industry and civilian nuclear industry policy since the 1970s had clearly been geared towards keep herself just one screw driver turn away a massive break out as a major nuclear weapon power With ready made strategic missile capability.